Precision resistors are used in many fields of application in electrical engineering and are well-known from the prior art. Precision resistors are usually produced by means of metal films of defined length and cross-sectional area. The current trend is to use predominantly metal films composed of nickel-chromium compounds or constantan. One drawback with a precision resistor made of a metal film lies in its significantly nonlinear, in particular parabolic, temperature dependence, which in general all alloys and metal exhibit.
Especially when nickel-chromium compounds are used for precision resistors, the electrical resistance also shows a dependence on the selected cooling rate during the production process, so that in limited temperature ranges a different temperature dependence of the electrical resistance can be achieved as a function of the production conditions. However, by applying defined annealing steps and cooling periods after the temperature treatments, it is possible in the case of such metal films to adjust the temperature coefficient of resistance to almost zero in a limited temperature range. While such metal film resistors can be assumed to be approximately temperature constant in the limited temperature range, they are, however, not useful as precision resistors over a wide temperature range, especially at temperatures below 200 K or at temperatures above 400 K.
Furthermore, it is known that carbon-containing films can be used as the resistive material. These materials exhibit a temperature coefficient of ±100 ppm/K in a limited temperature range and usually a high specific resistance of more than 1,000 μΩ·cm.
Coatings based on amorphous carbon a-C or hydrocarbon a-C:H respectively are sufficiently well-known from the prior art and are used primarily to reduce wear. Thus, DE 10 2006 029 415 and DE 10 2006 027 502 show suitable tribological functional layers, which reduce the friction and simultaneously increase the wear resistance of machine components.
It is also known that the electrical properties of carbon-containing films, in particular a-C:H films, can be significantly affected by the incorporation of metals. Thus, the German document DE 109 54 164 A1 proposes in order to adjust the ohmic resistance of carbon-containing films that the carbon-containing film be doped with a metal material, in order to suitably reduce the resistance. These thin films can be produced in a combined PVD/CVD [physical vapor deposition/chemical vapor deposition] process by sputtering from a target material, using an additionally introduced carbon-containing reactive gas. The result is the formation of metal clusters in a carbon matrix and/or hydrocarbon matrix that cannot be specified in detail.
Document EP 05 75 003 A2 also discloses a carbon-containing film that contains metal, the film being configured in such a way that no carbide forms between the metal and the carbon.
However, it has been observed that in the case of metal-containing amorphous hydrocarbons, in which the doping material contains, for example, one of the metals Ag, Au, Cu, Pt, or Pd, the temperature coefficients are unstable over a wide temperature range; that is, they change permanently, so that such a material is not suitable as a resistive material having defined and/or constant temperature coefficients.
Owing to the temperature dependence of their resistance, resistive films comprising a carbon-containing material are used, for example, as temperature sensors in stressed areas of machines. This feature is well known from document DE 102 53 178 A1, where a film of diamond-like carbon is used as a temperature sensor. However, the drawback in this case is the aforementioned nonlinear temperature dependence of the electrical resistance.
Similarly, the prior art discloses the piezo-resistive behavior of carbon in hydrocarbon films, because of which they are used as piezo-actuators or piezo-sensors. Hence, DE 10 2006 019 942 shows systems with amorphous carbon films, which exhibit piezo-resistive properties that can be used for measuring forces.
Owing to their piezo-resistive properties, the carbon-containing films have a high potential for sensor applications, such as for force or pressure sensors. While conventional strain gauges that are based on CrNi resistance structures exhibit a strain sensitivity in a range of a gauge factor (GF) approximately 2, which is caused by a geometric variation of the strain gauge, carbon-containing films, for example, amorphous carbon films, exhibit gauge factors in a range between 10 and 20. However, carbon-containing films also show, besides their piezo-resistive behavior, the above-described strong temperature dependence of their electrical resistance. Such a temperature dependence is disadvantageous when the piezo-resistive property of a carbon-containing film is used, for example, as a force sensor, because variations in the ambient temperature can lead to force-independent variations in resistance and, thus, to a distortion of the measurement results.
The object of the present invention is to provide a film resistor that exhibits a linear temperature sensitivity of its electrical resistance over a wide temperature range. Furthermore, the object of the present invention is to provide a method for producing a film resistor of this type.